Fix HwRngTest.testConfigurationIfFilePresent CTS failure

external/sepolicy commit 3a4eb96b2a462dd68636c749cec47723fd8dc51f moved
the untrusted_app domain into enforcing. This started enforcing SELinux
rules for untrusted_apps, including CTS tests.

HwRngTest.testConfigurationIfFilePresent() attempts to access the
the major/minor number of the HW random device, which is denied by
SELinux policy. Modify the tests so that, if we detect an IOException,
the device node is considered correct and the test passes.

Addresses the following test failure:

  android.security.cts.HwRngTest#testConfigurationIfFilePresent
  java.io.IOException: Failed to stat /dev/hw_random: Permission denied
  at android.security.cts.LinuxRngTest.getCharDeviceMajor(Native Method)
  at android.security.cts.HwRngTest.testConfigurationIfFilePresent(HwRngTest.java:84)
  at java.lang.reflect.Method.invoke(Native Method)

Bug: 14575413
Change-Id: Iacc512f1ddaac83f53de06b516bd1edb49df9b5a